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Destination Space

Why Are Plants Green?

Subject Area
Space Science
Estimated Classroom time
50 minutes
Difficulty
2 of 3 — Moderate

Why are plants green?

Main Theme: Cyanobacteria changed the colour of life on Earth and made complex life possible by filling the atmosphere with oxygen. They are the distant ancestors of every plant alive today. The Sun is the ultimate source of energy for nearly all life on Earth, and most of that energy is lost as it passes through a food chain.

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Session objectives

  • Explain why early Earth’s oceans were dominated by purple halobacteria, and how cyanobacteria evolved and caused the Great Oxidation Event.
  • State the word equation for photosynthesis and explain what each part represents.
  • Explain what a food chain is and trace the Sun’s energy through one example.
  • Explain why most energy is lost at each step in a food chain, and why this limits its length.
  • Identify and correct common misconceptions about bacteria, plants and energy transfer.

Misconceptions to watch for

  • All bacteria are harmful.

    Cyanobacteria are bacteria, and they are directly responsible for the oxygen in the air you are breathing right now. Many bacteria are essential to life on Earth.

  • The top predator in a food chain gets the most energy.

    The opposite is true. The top predator gets the least energy of anyone in the chain, because only around 10% passes on at each step. This is exactly why food chains cannot have many links.

If a student asks…

Do plants get their food from the soil?
lants make their own food through photosynthesis, using sunlight, water and carbon dioxide. Soil provides water and minerals, not food in the energy sense.
Do plants create energy?
Plants do not create energy from nothing. They capture and convert energy that already exists in sunlight, storing it as chemical energy in sugar. This is energy transfer, not energy creation.

Got longer?

Teacher Tips

Hook

  • It helps to give the full 60 seconds of silent looking before asking anything. The strangeness of a purple Earth needs to land on its own, so it’s worth not explaining the image straight away.
  • You could circulate during the pair talk (why plants are green) and listen for what learners guess before revealing halobacteria: different chemicals, different sunlight, an alien planet. It tells you what they already half know.
  • It’s worth not giving away what happened 2.7 billion years ago in the bridge. Let it stay a mystery they’re solving in the pathway, rather than something you’ve already told them.

ITZA Pathways

  • On Pathway 1, you might ask learners to explain in one sentence why cyanobacteria look green. If they can only say “because they’re plants”, that’s the one to go back to.
  • On Pathway 2, you could ask learners to trace the Sun’s energy through their own breakfast this morning. It makes the abstract food chain personal and concrete.
  • “Which fact surprised you most?” works well here too.

Classroom Activity

  • You can run this with learners physically moving to labelled corners, or as a show of hands if you’d rather keep them seated. Both work, and the corners version just makes disagreement more visible.
  • It helps not to confirm or deny an answer straight away. Asking “Can anyone convince them to move?” first is what creates the debate, rather than a quick right or wrong.
  • It’s worth getting learners’ reasoning out loud before you reveal the answer on the slide. The justification is doing more work than the verdict.
  • “Plants create energy” and “oxygen is a waste product” are the two most counterintuitive statements. Expect genuine disagreement, and let it run a little before resolving it.

Plenary

  • It helps to ask for a specific reason in the reflective answer: oxygen, the ozone layer, life on land, not just “everything would be different”.
  • The lock-in is from Michael Pollan, an American author and journalist best known for writing about food. He’s saying that the energy in your meal started out as sunlight, which is the food chain idea from today. It’s short, so it works to read it just as it is, then ask “Where did the energy in your last meal come from?”
Learner journal tip

If using the editable journal, it helps to have each learner make their own copy at the start of the course and save it somewhere easy to find again. Perhaps in a folder on their laptop or in their school drive. It carries across every session, so if the plenary runs short, they can finish Part 2 or the later sections at home without losing earlier reflections.

Extensions

  • A small dark brown mammal with a long, pointed pink snout, standing on a mossy log against a soft green background.

    Extension 1

    Meat Eating Plants

    Plants usually end up on the menu. But some have turned the tables, and now they’re the ones doing the eating.

    Explore(opens in a new tab)
  • Three dry, brown, curled-up plants sitting on cracked, sandy desert ground.

    Extension 2

    The Resurrection Planet

    In Africa’s Sahara desert, one plant has learned to cheat death. See how it does it.

    Explore(opens in a new tab)

Curriculum links

Biology
Photosynthesis as the basis of all food chains. Energy transfer through trophic levels. Cellular evolution and the role of cyanobacteria in shaping Earth's atmosphere.
Chemistry
The word equation for photosynthesis. Earth's early atmosphere and its transformation through biological processes.
Physics
Light absorption and reflection, and why objects appear the colour they do.
Geography
How biological processes have shaped Earth's atmosphere and made land habitable for animal life.
English & literacy
Evaluating statements and constructing a reasoned argument for agreement or disagreement.